134 A. H. Graves, 
on this condition in Lemna, say, “To divide a large sporogenous 
mass by sterile plates for better nutrition is too common to call 
for special remark.” As mentioned by Caldwell and Goebel 
(1898, p. 770), Isoetes presents a similar condition of formation of 
sterile plates of tissue from a fairly large archesporial mass.! 
The archesporial cells, therefore, now appear as two definite, 
rounded, densely staining masses, composed of sporogenous cells, 
surrounded by a primary parietal layer, which has undergone a 
periclinal division in two or three places. 
The first periclinal divisions in the primary parietal layer have 
become more general in the next stage, Pl. XI, fig. 73, which is from 
a flower about 0.33 mm. in length. The septum between the two 
sacs is also more conspicuous, and divisions continue in the sporo- 
genous tissue. 
At a considerably later period, with the length of the flower 
about 0.5 mm. ‘Pl. XI, fig. 74), the parietal layers are still two or 
occasionally three in number. Indications appear here that the ta- 
petum is forming from the marginal sporogenous tissue. Nuclear 
divisions continue among the sporogenous cells. 
Soon after this stage, however, the sporogenous cells attain their 
final number, and all division ceases, followed by an enlargement 
to the mature pollen mother-cells, just as Murbeck (1902) has re- 
corded for Ruppia rostellata. 
Pl. XI, fig. 75 shows how the tapetal cells, now unmistakable in 
form and structure, bound the sporogenous cells-—which may now 
be termed the pollen mother-cells—and are undoubtedly derived 
from them. According to Rosenberg (1901, IL), Zostera also forms 
tapetum from the ends of its long sporogenous cells, and Coulter 
and Chamberlain (1903. p. 37) have shown that this is not unusual 
nor unnatural. In this respect, together with the number of chro- 
mosomes in the dividing sporogenous cells, which I have found to 
be 16, and also the three or four parietal layers between the epi- 
dermis and tapetum, R. maritima corresponds exactly with R. ro- 
stellata, as described by Murbeck (1902, p. 5). It is interesting to 
note here that Wiegand (1899, pp. 344 and 345), finds in Potamogeton 
fohosus that the tapetum is “differentiated from the wall rather 
than from the archesporium.” 
There remain to be mentioned the dissolution of the tapetal cells. 
(Pl. XI, fig. 76), the development of thickenings in the subepidermal 
layer, and the final dehiscing of the anther by a longitudinal split. 
1 A like situation has been carefully described by Bower (1897, pp. 41 ff.), 
for Danaea and other Marattiaceae. 
